linux-kernel - Re: [PATCH 06/10] ksm: identify PageKsm pages

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Message-ID: <20090721175139.GE2239@random.random>
Date:	Tue, 21 Jul 2009 19:51:39 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Izik Eidus <ieidus@...hat.com>
Cc:	akpm@...ux-foundation.org, hugh.dickins@...cali.co.uk,
	chrisw@...hat.com, avi@...hat.com, riel@...hat.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	nickpiggin@...oo.com.au, Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH 06/10] ksm: identify PageKsm pages

On Fri, Jul 17, 2009 at 08:30:46PM +0300, Izik Eidus wrote:
> +static inline int PageKsm(struct page *page)
> +{
> +	return ((unsigned long)page->mapping == PAGE_MAPPING_ANON);
> +}

I'm unconvinced it's sane to have PageAnon return 1 on Ksm pages.

The above will also have short lifetime so not sure it's worth it,
if we want to swap we'll have to move to something that to:

PageExternal()
{
	return (unsigned long)page->mapping & PAGE_MAPPING_EXTERNAL != 0;
}

> +static inline void page_add_ksm_rmap(struct page *page)
> +{
> +	if (atomic_inc_and_test(&page->_mapcount)) {
> +		page->mapping = (void *) PAGE_MAPPING_ANON;
> +		__inc_zone_page_state(page, NR_ANON_PAGES);
> +	}
> +}

Is it correct to account them as anon pages?

> -	if (PageAnon(old_page)) {
> +	if (PageAnon(old_page) && !PageKsm(old_page)) {
>  		if (!trylock_page(old_page)) {
>  			page_cache_get(old_page);
>  			pte_unmap_unlock(page_table, ptl);

What exactly does it buy to have PageAnon return 1 on ksm pages,
besides requiring the above additional check (that if we stick to the
above code, I would find safer to move inside reuse_swap_page).
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